Thanks
I found various posts but not one place where all the information has been gathered or whether consensus had been reached.
I'll plan to take one of the audio outputs from the Pi. I had assumed I'd get better output by summing L&R, perhaps using a simple mono output 3.5mm jack (
https://cpc.farnell.com/pro-signal/psg08686/3-5mm-jack-plug-mono-gold-black/dp/CN21230 ), but I see that might not be the best.
Most intercoms have some way to include an aux input of some sort, traditionally to feed in marker tones (not that there are many of those around) or similar.
The SPA-400 involves adding a resistor, then linking it into the headphone output
https://www.sigtronics.com/air/spa.htmlhttps://www.sigtronics.com/_pdf/air_pdf/air_intercom_pdf/SPA-400_AUX_Audio_Input_Inst.pdfhttps://www.sigtronics.com/_pdf/air_pdf/air_intercom_pdf/ICS-Load_1440_Mod.pdfThe Trig Compact COMs have, like most modern radios, a built in Aux input:
https://www.trig-avionics.com/product/compact-radio/https://www.trig-avionics.com/library/00839-00-AG%20TY9x%20Installation%20Manual.pdf (search for Aux)
I'm unsure whether the audio output from the Pi will be strong enough for the two headsets that we have wired in via the SPA-400 and unsure of the resister value required to add to link it into that system to allow the intercom to still work. I was hoping that someone may have already tried it.
I'm more confident that input via the Trig will just work.
Adding a muting facility is important to us as both aircraft are used for circuit work and training from time to time. I don't want continuous bearingless information for someone taxiing around an airport outputting Mode-C while we are doing circuits under ATC control, yet bearingless is going to be important in the areas we normally operate in as a lookout reminder as I don't think there are enough ground stations for consistent multi-lateration (based at Prestwick). An external volume control, to me, is better than an on/off toggle.
I want an on/off switch as PAW will be installed behind the panel in each aircraft. I expect it will be kept off until engine has been started (we don't have separate avionics buses) to protect it and ensure max power is available for engine start (I know that modern avionics are designed to handle the power fluctuations during start but I suspect a Pi hasn't been?). A single knob seems a sensible way to do this and describe to the other large number of group members. I assume there isn't an audio annunciation that PAW is up and running? Could that be added as an enhancement so a pilot knows the system is working? A number of our group rarely use tablets (airspace isn't as congested up here!) so their only PAW interaction will be via audio.
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